Distributed spatial mode processing for spatial-mode multiplexed communication systems

ABSTRACT

A method and system for distributed spatial mode processing is disclosed where a plurality of optical signals is received via a plurality of spatial modes on a first optical link, spatial mode conversion is performed on the plurality of optical signals to switch the plurality of optical signals to different ones of the plurality of spatial modes and the plurality of optical signals is transmitted via the different ones of the plurality of spatial modes on a second optical link where spatial mode filtering may occur.

TECHNICAL FIELD

The present disclosure relates to optical communication systems and inparticular to systems and methods for improving the transmissionperformance of optical communication systems that transmit signals viamultiple spatial modes.

BACKGROUND

Optical communication systems are systems in which data is transmittedas light over optical fibers. It is widely recognized that single-modefiber capacity may soon approach a limit imposed by fiber nonlinearity,called the nonlinear Shannon limit. Since time-, wavelength-, andpolarization-division multiplexing have all been utilized already,recently the research community has started to explore the spatialdimension, utilizing so-called space-division multiplexing (SDM), inorder to further increase the fiber capacity to meet ever-growingcapacity demand.

A mode of an optical fiber is a self-consistent, transverse intensityprofile that maintains its shape as the light propagates down the fiber.An optical fiber has only a finite number of guided propagation modes,the intensity distributions of which have a finite extent around thefiber core. The number of guided modes, their transverse amplitudeprofiles, and their propagation constants depend on the details of thefiber structure (i.e. core and cladding diameters and core and claddingrefractive indices) and on the optical frequency. A single-mode fibersupports only a single guided mode per polarization direction, thelowest-order mode (LP₀₁), which has an intensity profile similar to thatof a Gaussian beam. SDM can be realized by several possible methods. Oneof the methods for space-division multiplexing includes transmittingmultiple independent signals over different spatial modes of amulti-mode or few-mode fiber. Other methods for space-divisionmultiplexing include transmitting multiple signals over multiplesingle-mode cores in an “uncoupled” (actually very weakly coupled)multi-core fiber or over super-modes in a coupled multi-core fiber.

However, for an SDM system based on few-mode fiber or coupled-multi-corefiber, there usually exists large inter-modal dispersion between theorthogonal spatial modes (except for the degenerate modes). Also,non-ideal fiber manufacturing and the cabling process, as well asaccidental fiber bending, may result in mode coupling when the signalpropagates down the fiber.

In order to properly de-multiplex the signal at the receiver, amulti-input-multi-output (MIMO) adaptive filter would be required. For atypical long-haul transmission system, prohibitively long tap lengthswould be required (i.e., tens of thousands of taps typically would berequired for just a three-mode fiber). Moreover, the signals transmittedon different spatial modes in a multi-mode fiber (or a coupledmulti-core fiber) may experience different amounts of loss. Suchmode-dependent loss will also limit the overall fiber capacity. For anSDM system using “uncoupled” multi-core fibers, MIMO processing is notcommonly used for spatial mode separation at the receiver. However, amode-dependent crosstalk still may be detected in the SDM system using“uncoupled” multi-core fibers. For example, for a regular 7-core fiber,crosstalk in the center core will be significantly higher than in the6other cores. For the case of 7-core fiber, the center core has worsetransmission performance, which limits the fiber capacity.

SUMMARY

In one embodiment a plurality of optical signals is received via aplurality of spatial modes on a first optical link, spatial modeconversion is performed on the plurality of optical signals to switchthe plurality of optical signals to different ones of the plurality ofspatial modes, and the plurality of optical signals is transmitted viathe different ones of the plurality of spatial modes on a second opticallink and the plurality of spatial modes is filtered during transmissionalong the second optical link, and performing spatial mode conversion onthe plurality of optical signals to switch the plurality of opticalsignals to different ones of the plurality of spatial modes comprises:switching an optical signal received on each of the plurality of modesin the first optical link to a different one of the plurality of modesin the second optical link, wherein one of the plurality of spatial modeconverters includes a Reconfigurable Optical Add-Drop Multiplexer,wherein the receiving a plurality of optical signals via a plurality ofspatial modes on a first optical link, performing spatial modeconversion on the plurality of optical signals to switch the pluralityof optical signals to different ones of the plurality of opticalsignals, and transmitting the plurality of optical signals via thedifferent ones of the plurality of spatial modes on a second opticallink are performed by a Reconfigurable Optical Add-Drop Multiplexer,wherein performing spatial mode conversion comprises: implementingrandom spatial mode scrambling in the second optical link due topredetermined bends in the second optical link. The said spatial modesused for transmitting the signal can be regular modes in a multi-mode(or few-mode) fiber, super modes in a strongly coupled multi-core fiber,or the fundamental mode of each individual single-mode core in an“uncoupled” multi-core fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary space-division multiplexing (SDM) andwavelength division multiplexing (WDM) transmission system, inaccordance with an embodiment;

FIGS. 2 a-2 f illustrate exemplary variations of the SDM and WDMtransmission media, in accordance to an embodiment;

FIG. 3 illustrates a method for distributed spatial mode processing,according to an embodiment;

FIGS. 4A-4B illustrate examples of distributed spatial mode conversionin a two-mode fiber, in accordance with an embodiment

FIG. 5 illustrates an exemplary functional block diagram for a spatialmode converter, in accordance with an embodiment;

FIG. 6 illustrates an exemplary functional block diagram for a spatialmode converter for a mode-group-by-mode-group spatial mode conversion,in accordance with an embodiment;

FIG. 7 illustrates an SDM system including a SMC/ROADM, in accordancewith an embodiment;

FIG. 8 illustrates an example of spatial mode filtering, in accordancewith an embodiment;

FIG. 9 illustrates an example in which distributed mode conversion anddistributed mode filtering are used together, in accordance with anembodiment;

FIGS. 10-12 illustrate exemplary methods for distributed spatial modeconversion, in accordance with various embodiments; and

FIG. 13 illustrates a high-level block diagram of a computer capable ofimplementing an exemplary method and system for distributed spatial modeconversion.

DETAILED DESCRIPTION

The present disclosure relates to methods and systems for distributedspatial mode processing in an optical communication system. Functionsand techniques performed by systems for distributed spatial modeprocessing are described in detail with further references to theexamples of FIGS. 1-13.

In an embodiment, a method and system for distributed spatial modeprocessing is described. The method and system for distributed spatialmode processing can be utilized to effectively mitigate the detrimentalinter-modal dispersion, mode-dependent loss, and inter-mode crosstalk byintroducing distributed mode conversion (or mode scrambling) along thefiber link. It is to be understood that, where only a portion of theavailable spatial modes are used for transmitting the signal,distributed spatial mode filtering can be used to reduce the detrimentalmodal crosstalk. The foregoing examples are to be understood as being inevery respect illustrative but not restrictive.

FIG. 1 illustrates an exemplary space-division multiplexing (SDM) andwavelength division multiplexing (WDM) transmission system, inaccordance to an embodiment. The exemplary system of FIG. 1 includes aSDM-WDM transmitter 100, a first nominal span 114, a second nominal span116, and a SDM-WDM receiver 120. The SDM-WDM transmitter 100 can includeone or more SDM transmitters 102 communicatively coupled to one or moreWDM multiplexers 104. In an embodiment, the SDM-WDM transmitter 100 canbe used to multiplex and transmit a plurality of optical signals ontotransmission medium 106. It is to be understood that the SDM-WDMtransmitter 100 may have other structures and may contain othercomponents as well, and that the SDM-WDM transmitter 100 is a high levelrepresentation of a SDM-WDM transmitter for illustrative purposes. Thefirst nominal span 114 includes fiber-based SDM-WDM transmission medium106 communicatively coupled to SDM-WDM amplifier 110 for amplifyingoptical signals to compensate the transmission loss of signal. Thesecond nominal span 116 includes fiber-based SDM-WDM transmission medium108 communicatively coupled to a reconfigurable optical add/dropmultiplexer (ROADM) 112 for adding and dropping individual opticalsignals. It is to be understood that the fiber-based SDM-WDMtransmission medium 108 can also be communicatively coupled to anotherSDM-WDM amplifier. The SDM-WDM receiver 120 can include one or more SDMreceivers 124 communicatively coupled to one or more WDM demultiplexers122. In an embodiment, the SDM-WDM receiver 120 can be used todemultiplex the plurality of optical signals received via transmissionmedium 106 and 108 and further process demultiplexed plurality ofoptical signals by the SDM receiver 120. It is to be understood that theSDM-WDM receiver 120 may have other structures and may contain othercomponents as well, and that the SDM-WDM receiver 120 is a high levelrepresentation of a SDM-WDM receiver for illustrative purposes. For asystem illustrated in FIG. 1, each single wavelength can be used totransmit multiple independent optical signals using several orthogonalspatial modes supported by fiber-based SDM and WDM transmission mediums106 and 108. It is to be understood that optical signals can beamplified at a regular distance interval (e.g., every 50-100 km) oftransmission to compensate the transmission loss.

FIGS. 2 a-2 f illustrate exemplary variations of the SDM and WDMtransmission media, in accordance to an embodiment. FIG. 2 a illustratesa conventional single-mode fiber 200. The single-mode fiber 200 includesa single core 202 surrounded by cladding 204. The core 202 provides asingle spatial mode for signal transmission. In some cases, 12single-mode fibers, each with 125 um cladding diameter and individuallycoated, are laid out in a row and then attached using a ribbon coatinglayer. While conventional single-mode fiber ribbons can serve as analternative for an SDM transmission medium, in the single-mode fiberribbons there is essentially no crosstalk between the cores.

FIG. 2 b illustrates exemplary multi-mode/few-mode fiber 210. Themulti-mode/few-mode fiber 210 has a single core 212 surrounded bycladding. In the multi-mode/few-mode fiber 210, the diameter of the core212 is enlarged, as compared to the conventional single-mode fiber 200of FIG. 2 a, to support more than one transverse mode.

FIG. 2 c illustrates an exemplary “uncoupled” multi-core fiber 220. Inthe “uncoupled” multi-core fiber 220, each core 222 a-222 g provides aseparate spatial mode for signal transmission. Cores 222 a-222 g areseparated from each other by a distance (i.e. the core pitch) that issignificantly larger than the core diameter for each core 222 a-222 g inorder to reduce inter-core mode coupling to a negligible level (to avoidthe need for MIMO processing at the receiver).

FIG. 2 d illustrates an exemplary coupled multi-core fiber 230. Thecoupled multi-core fiber 230 includes multiple single-mode cores 232a-232 c. In the coupled multi-core fiber 230, the core pitch is reducedto increase the density of cores 232 a-232 c within the fiber. Each ofthe cores 232 a-232 c provides a single spatial mode for signaltransmission. Both multi-mode fiber 210 of FIG. 2 b and coupledmulti-core fiber 230 can achieve higher mode density than the“uncoupled” multi-core fiber 220 of FIG. 2 c. However, multi-mode fiber210 of FIG. 2 b and coupled multi-core fiber 230 require extra digitalsignal processing (DSP) at the receiver (e.g., MIMO processing) to“undo” the mode coupling occurring due to the imperfect fibermanufacturing process as well as accidental macro- and micro-bendingthat may occur when cabled and deployed in the real world.

FIG. 2 e illustrates an exemplary compound multi-core fiber 240 whichmay consist of multiple “isolated” (or “uncoupled”) groups of coupledmultiple cores 242 a-242 g. The term “coupled cores” means thatcore-to-core distance between individual cores within each multiple coregroup 242 a-242 g (i.e., the nominal core-to-core pitch) is small enoughsuch that the mode coupling within each multiple cores 242 a-242 g isstrong while the core-group-to-core-group distance is large enough suchthat the mode coupling between core groups 242 a-242 g is weak. Thecompound multi-core fiber 240 enables higher fiber capacity withrealistic CMOS capability.

FIG. 2 f illustrates an exemplary compound multi-mode multi-core fiber250 consisting of multiple “isolated”/“uncoupled” multi-mode/few-modecores 252 a-252 g. The diameter of each of the cores 252 a-252 g isenlarged to support more than one transverse mode. The distance betweeneach of the cores 252 a-252 g is large enough such that the modecoupling between cores 252 a-252 g is weak. Similar to the exemplarycompound multi-core fiber 240, the exemplary compound multi-modemulti-core fiber 250 enables higher fiber capacity with realistic CMOScapability.

The allowable number of spatial modes in a multi-mode fiber or a coupledmulti-core fiber is limited by a Complementary Metal-Oxide-Semiconductor(CMOS) processing capacity. To overcome this limitation, the compoundmulti-core 240 or the compound multi-mode multi-core fiber 250 designs,illustrated, respectively, in FIGS. 2 e and 2 f, can be used accordingto an advantageous embodiment.

For an SDM system using a multi-mode core or coupled multiplesingle-mode cores as is shown in FIGS. 2 b, 2 d, and 2 e, signalstransmitted via different spatial modes typically travel at differentspeeds (except for the degenerate modes), which can result in largeinter-modal dispersion (e.g., the inter-mode dispersion between the LP01mode and LP11 mode is typically greater than several thousands ofpsec/km in a three-mode fiber). Large inter-mode dispersion coupled withunavoidable mode coupling requires the use of prohibitively long MIMOfilters at the receiver.

In accordance with an embodiment, the required MIMO filter length can besubstantially reduced by using the methods and systems for distributedmode conversion or mode-mixing described herein. Multiple discretespatial mode converters (SMCs) or spatial mode scramblers (SMSs) can beimplemented along the fiber link for distributed mode conversion ormode-mixing.

FIG. 3 illustrates an exemplary method 300 for the transmissionperformance of optical communication systems transmitting signals viamultiple spatial modes, according to an embodiment. At step 302, aplurality of optical signals can be received by a spatial mode converter(SMC) or spatial mode scrambler (SMS) via a first optical link. It is tobe understood that optical link can be a fiber of various configurationand/or structure. At step 304, distributed spatial processing of theplurality of optical signals can be performed by one or more SMC/SMS. Inone embodiment, distributed spatial processing can be performed byswitching modes (i.e., mode conversion) of the plurality of opticalsignals among available cores within the optical link (e.g., fiber). Itis to be understood that switching modes of the plurality of opticalsignals among available cores can include switching modes of each of theplurality of optical signals or switching modes of some of the pluralityof optical signals. In an embodiment, switching modes of the pluralityof optical signals among available cores can include switching modes ona group-by-group basis such that a group of modes of a plurality ofoptical signals is switched with another group of modes of a pluralityof optical signals.

In another embodiment, distributed spatial processing can be performedby distributed spatial mode filtering of optical signals from one ormore cores along the optical link to improve a performance of the SDMsystem by preventing or minimizing crosstalk between the optical signalsbeing transmitted via the optical link. It is to be understood thatdistributed spatial mode filtering can include filtering of one or moreoptical signals along the optical link. It is also to be understoodthat, to improve a performance of the SDM system, distributed spatialmode processing can include a combination of distributed spatial modeconversion and distributed spatial mode scrambling along the opticalfiber.

At step 306, the plurality of optical signals can be transmitted viasecond optical link of the fiber.

In accordance with an embodiment, the mode mixing or conversion can alsobe incorporated within the transmission fiber itself (e.g., byintentionally introducing fiber bending while drawing the fiber or incabling, as illustrated on FIG. 12).

It is to be understood by a person ordinarily skilled in the art thatthe spatial mode conversion can be implemented by utilizing severaltechniques and methods, such as fiber-grating-based technique, LiquidCrystal on Silicon (LCoS) method, or space optics method.

FIGS. 4A and 4B illustrate examples in which a two-mode fiber is used totransmit two data signals: data A and data B, over a fiber length ofL=100 km. Specifically, FIG. 4A illustrates a conventional SDM system400 in which one of the two spatial modes is used to transmit data Aover the whole fiber length L and the other spatial mode is used totransmit data B over the whole fiber length L of the SDM system 400.Assuming that the inter-mode dispersion coefficient D_(m) between mode Aand mode B is 4 ps/m, at the receiver, the accumulated mode dispersionτ_(m) or walk-off between signals A and B is 400,000 ps for a fiberlength of L=100 km.

FIG. 4B illustrates an SDM system 420 in which an SMC 410 is placed inthe middle of the 100 km fiber of SDM system 420 where in the first 50km mode A transmits data A and mode B transmits data B, while in thesecond 50 km the SMC 410 will cause mode B to transmit data A and mode Ato transmit data B. In this case, after 100 km of transmission, the netwalk-off or mode dispersion τ_(m) between data A and data B will bezero, assuming that no mode coupling occurs during transmission. Oneskilled in the art will understand that, in more realistic case wheremode coupling does occur along the fiber, the net modal dispersion τ_(m)may not be zero but will be smaller than in the case illustrated in FIG.4A. It is to be understood that the net modal dispersion τ_(m) can bereduced even further if more than one SMC is placed along the fiber link(e.g., an SMC can be placed at every splice, which typically occur every3 to 5 km).

FIG. 5 illustrates an exemplary functional block diagram for a spatialmode converter (SMC) 500 where spatial mode conversion is performed on amode-by-mode basis, according to an embodiment. In an embodiment, theSMC 500 can include a spatial mode demultiplexer 502 communicativelycoupled to a spatial mode switch matrix 504, which, in turn, iscommunicatively coupled to a spatial mode multiplexer 506. The spatialmode multiplexer 506 is used to multiplex a plurality of optical signalsonto a single optical fiber by using a plurality of modes to transmitthe plurality of optical signals, thus increasing optical fibertransmission capacity. The spatial mode demultiplexer 502 is a devicetaking a single input fiber that carries one or more signals on aplurality of modes of a fiber and separates those signals over multiplemodes in separate fiber outputs. The spatial mode switch matrix 504 is adevice that performs a mode conversion based on a number of factors,such as type (e.g., single-mode, multi-mode, “uncoupled” multi-core,“coupled” multi-core, etc.) of optical fiber in the SMC 500. In anembodiment, a mode conversion is switching of the optical signal fromone of a plurality of spatial modes onto one other of the plurality ofspatial modes within the optical fiber. It is to be understood that thespatial mode converter 500 can also be used to enable bidirectionalcommunication.

In an advantageous embodiment, the input of the SMC 500 receives anoptical signal via a first optical link of a fiber. The optical signalis demultiplexed by taking the optical signal, separating the signalsbeing transmitted on the various modes, and selecting one of one or moredata-output-lines of the spatial mode demultiplexer 504 to transmit oneor more of the signals on the various modes. Demultiplexed opticalsignal is then transmitted to the spatial mode switch matrix 504 for amode conversion. Subsequently, converted optical signal is transmittedto the spatial mode multiplexer 506 to be transmitted via a secondoptical link.

In an embodiment, the spatial mode switch matrix 504 determines a modeconversion in the SMC 500 in a deterministic way, i.e., upon processingof optical signals received from the spatial mode demultiplexer 502, thespatial mode switch matrix 504 performs spatial mode conversion inaccordance with pre-defined algorithms and based on a type of theoptical link used for transmission of the optical signal. In anotherembodiment, mode conversion can be performed by the spatial mode switchmatrix 504 in a random way.

FIG. 6 illustrates an exemplary functional block diagram for a spatialmode converter (SMC) 600 where spatial mode conversion is performed on amode-group-by-mode-group basis, according to an embodiment. In anembodiment, the SMC 600 can include a spatial mode demultiplexer 602communicatively coupled to a spatial mode switch matrix 604, which, inturn, is communicatively coupled to a spatial mode multiplexer 606. Thespatial mode multiplexer 606 is used to multiplex a plurality of opticalsignals onto a single optical fiber by using a plurality of mode groupsto transmit the plurality of optical signals, to increase optical fibertransmission capacity. The spatial mode demultiplexer 602 is a devicetaking a single input signal that carries one or more signals on aplurality of modes groups of a fiber and separates those signals overmultiple mode groups in separate fiber outputs. The spatial mode switchmatrix 604 is a device that performs a mode conversion based on a numberof factors, such as type (e.g., single-mode, multi-mode, “uncoupled”multi-core, “coupled” multi-core, etc.) of optical fiber in the SMC 600.In an embodiment, a mode conversion is switching of the optical signalfrom one group of a plurality of spatial modes onto one other group ofthe plurality of spatial modes within the optical fiber. It is to beunderstood that the spatial mode spatial mode converter (SMC) 600, wherespatial mode conversion is performed on a mode-group-by-mode-groupbasis, can also be used to enable bidirectional communication.

In an advantageous embodiment, the input of the SMC 600 is an opticalsignal via a first optical link of a multi-mode fiber. The opticalsignal is demultiplexed by taking the optical signal and separating thesignals being transmitted on the various modes, and selecting one of oneor more groups of data-output-lines of the spatial mode demultiplexer604 to transmit one or more of the signals on the various mode groups.Demultiplexed optical signal is then transmitted to the spatial modeswitch matrix 604 for a mode conversion. Subsequently, converted opticalsignal is transmitted to the spatial mode multiplexer 606 to betransmitted via a second optical link.

In an embodiment, the spatial mode switch matrix 604 determines a modeconversion in the SMC 600 in a deterministic way, i.e., upon processingof optical signals received from the spatial mode demultiplexer 602, thespatial mode switch matrix 604 performs spatial mode conversion inaccordance with pre-defined algorithms and based on a type of theoptical link used for transmission of the optical signal. In anotherembodiment, mode conversion can be performed by the spatial mode switchmatrix 604 in a random way.

Those skilled in the art will understand that a first optical link and asecond optical link can be any type of fiber illustrated in FIGS. 2 b-2f. It is to be understood that discrete SMCs or SMSs can be included inthe optical amplifiers and/or ROADMs that are placed between spans.Those skilled in the art will understand that a mode conversion by SMCsand SMSs can be repeated a number of times throughout the transmissionof the optical signal to compensate detrimental inter-modal dispersion,mode-dependent loss, and inter-mode crosstalk during transmission.

In accordance with an embodiment, the optical amplifiers and/or ROADMsin an SDM system can provide a natural place to incorporate SMC becausethe optical signals from the multiple cores or multiple modes likelywill be in free space for gain equalization or switching purposes. FIG.7 illustrates an SDM system which includes a SMC/ROADM 710 which issituated between a 7-core fiber 702 and a 7-core fiber 704. TheSMC/ROADM 710 switches a signal carried by a wavelength on each of the 7cores of fiber 702 to a different one of the 7 cores of fiber 704. Forexample, SMC/ROADM 710 switches the optical signal carried in core 706of fiber 702 to core 708 of fiber 704. The signal in each core of fiber702 is switched to a different core of fiber 704, thus achievingdiscrete mode conversion. With the use of distributed mode conversion ormixing, the spatial mode used for transmitting a particular signalchanges from fiber segment to segment. As a result, the accumulatedinter-modal dispersion experienced by the signal decreases due toaveraging.

According to an embodiment, the use of distributed mode conversion ormixing also can reduce mode-dependent loss as the signal is transmittedvia different spatial modes along the fiber and thus the average lossamong a plurality of transmitted signals at the end of transmission willbe close to identical. For an SDM system using only “uncoupled”multi-core fiber, where no MIMO processing is used at the receiver,core-to-core crosstalk may become a problem for the cores having highestcrosstalk (e.g., the center core for a typical seven-core fiber of FIG.2 c) as such crosstalk increases linearly with transmission distance.

According to an embodiment, the problem of the cores having highcrosstalk due to large transmission distance can be substantiallymitigated by the proposed distributed mode conversion method in which aparticular signal is transmitted over different cores from one fibersegment to another segment, from one span to another span, or from onelink to another link. As explained above, the mode conversion can beimplemented in network elements such as an optical amplifier and/orROADM. For an SDM system using a compound multi-mode, multi-core fiberor compound multi-core fiber, mode-by-mode conversion method can beapplied to each multi-mode core or each coupled-core group (to reducethe detrimental impact from inter-modal dispersion and mode-dependentloss), whereas the group-by-group mode conversion method can be used tomitigate crosstalk between multi-mode cores or between coupled coregroups.

In accordance with an embodiment, distributed spatial mode filtering canbe used to improve the performance for an SDM system using only aportion of the available spatial modes for signal transmission. FIG. 8illustrates an example of spatial mode filtering. As illustrated in FIG.8, the two fundamental modes of cores 810 and 820 are used to transmittwo independent signals (data A and data B) in a three core fiber 802,where the fundamental mode of core 830 is not used for signaltransmission.

During transmission, the signal energy can couple into the fundamentalmode of core 830 from the fundamental modes of cores 810 or 820, andfurthermore, the mode of core 830 can couple back into the modes ofcores 810 or 820, causing crosstalk. Such crosstalk can be effectivelymitigated by performing distributed spatial mode filtering, either byfiltering the fundamental mode of core 830 during transmission along thefiber or by adding multiple discrete spatial mode filters along thefiber link (e.g. at the optical amplifiers and/or ROADMs).

In accordance with an alternative embodiment, the fiber of FIG. 8 couldbe few-mode fiber, where two degenerate modes, the LP11 a and LP11 bmodes, are used to transmit two independent signals (data A and data B),and where fundamental mode (LP01) is not used for signal transmission.During transmission, the signal energy can couple into the fundamentalmode from the degenerate modes LP11 a and LP11 b, and furthermore, thefundamental mode can couple back into the degenerate modes, causingcrosstalk. Such crosstalk can be effectively mitigated by performingdistributed spatial mode filtering, either by filtering the fundamentalmode during transmission along the fiber or by adding multiple discretespatial mode filters along the fiber link (e.g. at the opticalamplifiers and/or ROADMs).

FIG. 9 illustrates an example in which distributed mode conversion anddistributed mode filtering are used together to improve the performanceof an SDM system. As shown in FIG. 9 an exemplary SDM system which usesa five-core fiber 900 as the transmission medium to transmit fourindependent signals over fundamental modes of cores 906, 908, 902, and904, while the fundamental mode of core 910 is not used for signaltransmission. In this case, distributed spatial mode filtering of thefundamental mode in core 910 can be used to reduce the crosstalk signalpresent in the fundamental modes of cores 906 and 908 resulting frommode conversion M from cores 906 to 910 to 906, 908 to 910 to 906, 906to 910 to 908, and 908 to 910 to 908. Similarly, distributed spatialmode filtering of the fundamental mode in core 910 can be used to reducethe crosstalk signal present in the fundamental modes of cores 902 and904 resulting from mode conversion N from cores 902 to 910 to 902, 904to 910 to 902, 902 to 910 to 904, and 904 to 910 to 904. Group-by-groupdistributed mode conversion S, where fundamental modes of cores 906 and908 are converted to fundamental modes of cores 902 and 904,respectively, and vice versa, can be used to reduce the impairment frominter-mode dispersion (between 906-908 and 902-904) and mode-dependentloss.

In accordance with another embodiment, distributed mode conversion anddistributed mode filtering are used together to improve the performanceof an SDM system based on multimode fiber. For example, for an SDMsystem using a five-mode fiber (with modes LP01 fundamental mode,degenerate modes LP11 a and LP11 b, and degenerate modes LP21 a and LP21b) as the transmission medium, four independent signals are transmittedover two modes LP11 a and LP11 b and two modes LP21 a and LP21 b, whilethe fundamental mode is not used for signal transmission. Distributedspatial mode filtering of the fundamental mode can be used to reduce thecrosstalk signal present in the LP11 a and LP11 b modes caused bycross-talk via mode conversions LP11 b-LP01-LP11 a, LP21 a-LP01-LP11 a,LP11 a-LP01-LP11 b, LP21 b-LP01_LP11 b, etc. Group-by-group distributedmode conversion of LP11 a to LP21 a and LP11 b to LP21 b and vice versacan be used to reduce the impairment from inter-mode dispersion betweenLP11 and LP21 modes and mode-dependent loss.

FIGS. 10-12 illustrate exemplary methods for distributed spatial modeconversion, in accordance with various embodiments. FIG. 10 illustratesexemplary SDM system which includes SDM-WDM transmitter 1002, SDM-WDMreceiver 1010, and one or more spans 1004, 1006, and 1008 of opticalfiber where each span 1004, 1006, and 1008 includes a spatial modeconverter/filter.

FIG. 11 illustrates exemplary SDM system which includes SDM-WDMtransmitter 1102, SDM-WDM receiver 1110, and one or more spans 1104,1106, and 1108 of optical fiber. As shown in FIG. 11, a ROADM isincluded in only some spans (1104 and 1108) of the spans 1104, 1106, and1108. For example, spatial mode converters and/or spatial mode filtersmay be included only in spans 1104 and 1108 including ROADM. It is to beunderstood that the number of spatial mode converter/filters situatedalong optical fiber can depend of a length of the optical fiber.

In an embodiment, inter-mode spatial conversion or mode mixing can beperformed in optical fiber itself. The inter-mode spatial conversion ormode mixing can be performed in optical fiber when, for example, bendsor long period gratings are induced in the fiber. The long period fibergratings along the fiber length may be achieved using a variety ofmethods, including squeezing the fiber against a periodic externalcorrugation structure, etching corrugated structures directly into thefiber cladding, generating index grating using acoustic waves, orproducing photo-induced index grating with intense ultra-violetradiation. FIG. 12 illustrates an exemplary SDM system which includesSDM-WDM transmitter 1202, SDM-WDM receiver 1210, and one or more fiberspans 1204, 1206, and 1208 of optical fiber, where each fiber span 1204,1206, and 1208 includes bends of optical fiber. In another embodiment,an exemplary SDM system can include SDM-WDM transmitter 1202, SDM-WDMreceiver 1210, and one or more fiber spans 1204, 1206, and 1208 ofoptical fiber, where each fiber span 1204, 1206, and 1208 includes longperiod gratings. It is to be understood that bends can be induced in theoptical fiber during the manufacturing of the optical fiber, during thecabling process, and/or during installation of the optical fiber withina signal communication infrastructure. It is also to be understood thatbends in the optical fiber can be embedded randomly or in accordance toa predetermined scheme.

One skilled in the art will recognize that methods of FIGS. 1, 3, 5, and6 are non-limiting and that components of the presented system may becombined in any way in various embodiments and may include anyadditional and/or desired components and/or configurations.

FIG. 13 is a high-level block diagram of an exemplary computer that maybe used for implementing distributed spatial mode processing. Computer1300 comprises a processor 1301 operatively coupled to a data storagedevice 1302 and a memory 1303. Processor 1301 controls the overalloperation of computer 1300 by executing computer program instructionsthat define such operations. The computer program instructions may bestored in data storage device 1302, or other computer readable medium,and loaded into memory 1303 when execution of the computer programinstructions is desired. Thus, methods of FIGS. 1, 3, 5, and 6 can bedefined by the computer program instructions stored in memory 1303and/or data storage device 1302 and controlled by processor 1301executing the computer program instructions. For example, the computerprogram instructions can be implemented as computer executable codeprogrammed by one skilled in the art to perform an algorithm defined bythe method of FIGS. 1, 3, 5, and 6. Accordingly, by executing thecomputer program instructions, the processor 1301 executes an algorithmdefined by the method steps of FIG. 3. Computer 1300 also includes oneor more network interfaces 1305 for communicating with other devices viaa network. Computer 1300 also includes one or more input/output devices1304 that enable user interaction with computer 1300 (e.g., display,keyboard, mouse, speakers, buttons, etc.).

Processor 1301 may include both general and special purposemicroprocessors, and may be the sole processor or one of multipleprocessors of computer 1300. Processor 1301 may comprise one or morecentral processing units (CPUs), for example. Processor 1301, datastorage device 1302, and/or memory 1303 may include, be supplemented by,or incorporated in, one or more application-specific integrated circuits(ASICs) and/or one or more field programmable gate arrays (FPGAs).

Data storage device 1302 and memory 1303 each comprise a tangiblenon-transitory computer readable storage medium. Data storage device1302, and memory 1303, may each include high-speed random access memory,such as dynamic random access memory (DRAM), static random access memory(SRAM), double data rate synchronous dynamic random access memory (DDRRAM), or other random access solid state memory devices, and may includenon-volatile memory, such as one or more magnetic disk storage devicessuch as internal hard disks and removable disks, magneto-optical diskstorage devices, optical disk storage devices, flash memory devices,semiconductor memory devices, such as erasable programmable read-onlymemory (EPROM), electrically erasable programmable read-only memory(EEPROM), compact disc read-only memory (CD-ROM), digital versatile discread-only memory (DVD-ROM) disks, or other non-volatile solid statestorage devices.

Input/output devices 1304 may include peripherals, such as a printer,scanner, display screen, etc. For example, input/output devices 1304 mayinclude a display device such as a cathode ray tube (CRT), plasma orliquid crystal display (LCD) monitor for displaying information to theuser, a keyboard, and a pointing device such as a mouse or a trackballby which the user can provide input to computer 1300.

One skilled in the art will recognize that an implementation of anactual computer or computer system may have other structures and maycontain other components as well, and that FIG. 13 is a high levelrepresentation of some of the components of such a computer forillustrative purposes.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are only forillustrative purposes and that various modifications may be implementedby those skilled in the art without departing from the scope and spiritof the invention. Those skilled in the art could implement various otherfeature combinations without departing from the scope and spirit of thedisclosure.

1. A method for distributed spatial mode processing comprising:receiving a plurality of optical signals via a plurality of spatialmodes on a first optical link; performing spatial mode conversion on theplurality of optical signals to switch the plurality of optical signalsto different ones of the plurality of spatial modes; and transmittingthe plurality of optical signals via the different ones of the pluralityof spatial modes on a second optical link.
 2. The method of claim 1,wherein the first and second optical link each include a plurality ofcores, and the performing spatial mode conversion on the plurality ofoptical signals to switch the plurality of optical signals to differentones of the plurality of spatial modes comprises: switching an opticalsignal received on each of the plurality of cores in the first opticallink to a different one of the plurality of cores in the second opticallink.
 3. The method of claim 1, wherein the first optical link and thesecond optical link each have a plurality of groups of coupled cores,and switching the plurality of optical signals from one of a pluralityof spatial modes onto one other of the plurality of spatial modescomprises: switching optical signals from each group of coupled cores inthe first optical link to a different group of coupled cores in thesecond optical link on a group-by-group basis.
 4. The method of claim 1,further comprising: filtering the plurality of spatial modes duringtransmission along the second optical link.
 5. The method of claim 1,further comprising: repeating the receiving a plurality of opticalsignals via a plurality of spatial modes on a first optical link,performing spatial mode conversion on the plurality of optical signalsto switch the plurality of optical signals to different ones of theplurality of optical signals, and transmitting the plurality of opticalsignals via the different ones of the plurality of spatial modes on asecond optical link at each of the plurality of spatial mode convertersalong a plurality of optical links.
 6. The method of claim 5, whereinone of the plurality of spatial mode converters includes aReconfigurable Optical Add-Drop Multiplexer.
 7. The method of claim 1,wherein the receiving a plurality of optical signals via a plurality ofspatial modes on a first optical link, performing spatial modeconversion on the plurality of optical signals to switch the pluralityof optical signals to different ones of the plurality of opticalsignals, and transmitting the plurality of optical signals via thedifferent ones of the plurality of spatial modes on a second opticallink are performed by a Reconfigurable Optical Add-Drop Multiplexer. 8.The method of claim 1, wherein performing spatial mode conversioncomprises: implementing random spatial mode scrambling in the secondoptical link due to predetermined bends in the second optical link. 9.An apparatus for distributed spatial mode processing comprising: amemory storing computer program instructions; and a processorcommunicatively coupled to the memory, the processor for executing thecomputer program instructions, which, when executed on the processor,cause the processor to perform operations comprising: receiving aplurality of optical signals via a plurality of spatial modes on a firstoptical link; performing spatial mode conversion on the plurality ofoptical signals to switch the plurality of optical signals to differentones of the plurality of spatial modes; and transmitting the pluralityof optical signals via the different ones of the plurality of spatialmodes on a second optical link.
 10. The apparatus of claim 9, whereinthe first and second optical link each include a plurality of cores, andthe performing spatial mode conversion on the plurality of opticalsignals to switch the plurality of optical signals to different ones ofthe plurality of spatial modes comprises: switching an optical signalreceived on each of the plurality of cores in the first optical link toa different one of the plurality of cores in the second optical link.11. The apparatus of claim 9, wherein the first optical link and thesecond optical link each have a plurality of groups of coupled cores,and switching the plurality of optical signals from one of a pluralityof spatial modes onto one other of the plurality of spatial modescomprises: switching optical signals from each group of coupled cores inthe first optical link to a different group of coupled cores in thesecond optical link on a group-by-group basis.
 12. The apparatus ofclaim 9, further comprising: filtering the plurality of spatial modesduring transmission along the second optical link.
 13. The apparatus ofclaim 9, further comprising: repeating the receiving a plurality ofoptical signals via a plurality of spatial modes on a first opticallink, performing spatial mode conversion on the plurality of opticalsignals to switch the plurality of optical signals to different ones ofthe plurality of optical signals, and transmitting the plurality ofoptical signals via the different ones of the plurality of spatial modeson a second optical link at each of the plurality of spatial modeconverters along a plurality of optical links.
 14. The apparatus ofclaim 13, wherein one of the plurality of spatial mode convertersincludes a Reconfigurable Optical Add-Drop Multiplexer.
 15. Theapparatus of claim 9, wherein the receiving a plurality of opticalsignals via a plurality of spatial modes on a first optical link,performing spatial mode conversion on the plurality of optical signalsto switch the plurality of optical signals to different ones of theplurality of optical signals, and transmitting the plurality of opticalsignals via the different ones of the plurality of spatial modes on asecond optical link are performed by a Reconfigurable Optical Add-DropMultiplexer.
 16. The apparatus of claim 9, wherein performing spatialmode conversion comprises: implementing random spatial mode scramblingin the second optical link due to predetermined bends in the secondoptical link.
 17. A tangible computer readable medium storing computerprogram instructions, which, when executed on a processor, cause theprocessor to perform operations comprising: receiving a plurality ofoptical signals via a plurality of spatial modes on a first opticallink; performing spatial mode conversion on the plurality of opticalsignals to switch the plurality of optical signals to different ones ofthe plurality of spatial modes; and transmitting the plurality ofoptical signals via the different ones of the plurality of spatial modeson a second optical link.
 18. The tangible computer readable medium ofclaim 17, wherein the computer program instructions which, when executedby the processor, cause the processor to perform further operationscomprising: filtering the plurality of spatial modes during transmissionalong the second optical link.
 19. The tangible computer readable mediumof claim 17, wherein the computer program instructions which, whenexecuted by the processor, cause the processor to perform furtheroperations comprising: repeating the receiving a plurality of opticalsignals via a plurality of spatial modes on a first optical link,performing spatial mode conversion on the plurality of optical signalsto switch the plurality of optical signals to different ones of theplurality of optical signals, and transmitting the plurality of opticalsignals via the different ones of the plurality of spatial modes on asecond optical link at each of the plurality of spatial mode convertersalong a plurality of optical links.
 20. The tangible computer readablemedium of claim 17, wherein the computer program instructions which,when executed by the processor, cause the processor to perform furtheroperations comprising: the receiving a plurality of optical signals viaa plurality of spatial modes on a first optical link, performing spatialmode conversion on the plurality of optical signals to switch theplurality of optical signals to different ones of the plurality ofoptical signals, and transmitting the plurality of optical signals viathe different ones of the plurality of spatial modes on a second opticallink are performed by a Reconfigurable Optical Add-Drop Multiplexer.